The present invention relates to a method for fabricating a plastic-lined weldment where welding is accomplished with the plastic liner in place, and to the plastic-lined weldment produced thereby.
In U.S. Pat. No. 4,522,432, issued June 11, 1985, on an application of Irving D. Press, and entitled "Components For Lined Piping System With Anchored Liners And Method Of Manufacture", assigned to the same assignee as the present application, there are disclosed, inter alia, tee and cross fittings which were produced by assembling metal stub ends over a preformed plastic liner, after which the joints between the stub ends were completed by welding. As explained in the patent in column 4, the stub ends were formed with the usual chamfered or beveled edge consisting of a root surface normal to the longitudinal axis of the stub end and a beveled surface. The patent text explained that if the weld is properly formed it should appear generally as shown in FIG. 6 of the patent which shows the weld essentially filling the vee-groove but with negligible root penetration. It was noted that preferably the weld is controlled or other means provided such that thermal degradation of the liner is minimized or avoided. The approach suggested in the patent was to ensure that the weld did not penetrate too far below the "break line", i.e., below the apex of the vee-groove; and the vee-groove had a depth about 50% of the thickness of the metal.
While fittings produced as disclosed in said patent have functioned satisfactorily in the field, and metal housings have not experienced failure in the weld areas, the present emphasis on safety has prompted search for a method by which greater weld penetration could be obtained. Weld penetration can be increased either by applying more welding heat to the joint or by modifying the joint geometry to provide a deeper groove or by a combination of both. Unfortunately, neither procedure can be utilized with heretofore known techniques without risk of causing thermal damage to the underlying liner. With both procedures increased heat reaches the plastic with detrimental effect. Moreover, the procedures are accompanied by increased outgassing of the metal both throughout the weld and from the adjacent base metal. In addition, some outgassing from the underlying plastic (e.g., PTFE) cannot be avoided. Any air trapped between the metal housing and the plastic liner tries to expand with the increase in temperature. All of these expanding gases tend to cause blowouts of the weld metal. Attempts to attain higher temperatures, if the weld metal does not blow out, results in weakening of the underlying plastic causing the latter to implode. Generally, however, blowout of molten metal occurs before implosion of the plastic, in either case relieving the pressure.
Whenever production of a plastic-lined welded metal component is considered, the usual question is why not fabricate the welded housing first, then mold the liner in situ? There are several objections to this procedure. Assuming that one could mold satisfactorily in situ, the finished liner cannot be inspected adequately for defects or imperfections. But in situ molding is otherwise unsatisfactory because of the differing thermal coefficients of expansion as between metal and plastic. Particularly with PTFE or similar resin the plastic will shrink back away from the wall of the housing as the two materials cool after molding. This results in a loose fitting liner. Also, where pipe fittings or piping is concerned, because the liner is flared over the several flange faces and the flares must be secured in place during molding, residual stresses are developed as the material shrinks upon cooling.
Preferably, as explained in the foregoing patent, the liners for the components are isostatically molded, and this is accomplished in precision molds outside of the eventual housing. This method is preferred for resins such as PTFE because the raw material cost for isostatic molding is less than that for extrusion molding.